The present invention relates to an apparatus for causing a controlled curved sawing or cutting of two-faced cants.
A two-faced cant is produced by removing from a log, either by sawing or chipping, an approximately equal volume from each side, after such log has been rotated in an angular position which places its natural curvature in a plane parallel to the two faces to be obtained in the process. Such cants are subsequently resawn perpendicularly to the cut faces to produce lumber of such dimensions as required in the marketplace.
While dimensional accuracy and edge straightness are requisites for this product, it has long been known that some bowing in a board across its thickness dimension, such as may occur in natural fiber stress relieving during the cut, is not detrimental to the quality of merchantable lumber since subsequent stacking and drying processes will straighten out any such curvature to a commercially acceptable degree.
In time, lumber producers have realized that straight line sawing along the length of a naturally curved two-faced cant could entail important losses in lumber yield. This led to a search for means to execute parallel curved cuts in a bowed workpiece, while preserving the thickness accuracy of every board produced. As long as cutting speeds were kept at low levels, simple ways including manual guiding were devised and used with some degree of success. However, during the past 20 years, sawing technology and market pressures have pushed feed rates beyond eight feet per second and new guiding methods for curved sawing had to be found.
Up to present times, most of the improved guiding systems which have been devised for this purpose still involve contact with the rough sides of the two-faced cants being processed. Even if such devices respond to the general curvature of the workpiece, surface deformities alter their perception of the basic shape of the piece. Besides, effective curve limiting in the cut is not possible because the guiding effect depends totally on contact with the natural surfaces of the piece, thus producing at times lumber that is bowed to such an extend as to cause problems in subsequent handling operations.
On the other hand, it is well known that, in making longitudinal cuts perpendicularly to the parallel planar faces of a two-faced cant, any angular misalignment of the feed rolls in contact with said planar faces will cause a deviation of the cut from a straight line. Angular misalignment in this context is considered as any departure from 90xc2x0 in the angle between the axis of said feed rolls and the theoretical feed line, as viewed in the plane parallel to the planar faces of the cant. A previous development, such as described in applicant""s U.S. Pat. No. 4,400,842, issued Mar. 28, 1995 to Brisson, has made use of this particularity basically by means of a pivotally mounted feed roll system which has variable orientation in the horizontal plane, thus exerting on the planar faces of the cant a lateral frictional force to cause a deviation of the workpiece during the cut. Since the intent of this development was to use a single guiding unit whether in the front or the back of the saws, it follows that a certain length of the cut, at one end or the other of the piece, is beyond curve control and remains straight. Also, the guiding action caused by the angular misalignment of feed rolls as described above, is dependent on friction factors which vary with the condition of the wood; in any situation, the actual deviation rate of the workpiece tends to be slow in relation to the total process cycling time of one to two seconds, depending on piece length and feed rate. This condition limits to a large extent reversed curve sawing in compound curvature cants.
It is an object of the present invention to provide a control system for the perpendicular resawing or facing of two-faced cants while following the longitudinal axis of their natural curvature in response to data obtained from a scanner upstream of the sawing or cutting unit.
Another object is to provide a control system for the perpendicular resawing or facing of two-faced cants through a guiding procedure which is entirely carried out by contact of mechanical components with the planar faces of the cant and therefore unaffected by any natural surface irregularities encountered on its rough sides.
A further object of the invention is to provide a curved sawing control system which is not influenced by any type of natural defects in a two-faced cant, thus eliminating the need for previous inspection and reject, while preventing undue saw stresses caused by sudden lateral feed line deviations which may occur in other systems using rough side contact for cant guidance.
Another object of the invention is to provide a system which can, for any condition of the cant, limit the degree of curving of the cut to whatever is deemed acceptable in sawn lumber with respect to subsequent handling and processing.
Another object is to provide a mechanical guide system for curved perpendicular resawing of two-faced cants which is mainly free of lateral slippage at the contact areas with the planar faces of the workpiece.
Another object is to provide a guiding system having definite lateral motion in either direction, such movements being controlled in speed and instantly reversible, thus allowing variable and compound curvatures of a cant to be reproduced in the cut, throughout the full length of the piece.
A further object is to provide a guiding system which causes a horizontally pivoting motion of the cant, about a vertical rotation axis whose preferred location, in or near the cut zone, can be selected by suitable design of control linkage components of the system.
Hence, one main object of this invention, which is to optimize lumber yield when resawing bowed cants, further requires that any movement of the guiding elements be computer programmed from a full form scan of the workpiece as it enters the system.
The present invention is concerned with a fast positively acting guiding system to be used in processing two-faced cants along the mean axis of their natural curvature, either in multiple resawing perpendicularly to the parallel planar faces, or in squaring off by means of a double facing canter. This is achieved by using laterally moveable infeed and ouffeed guiding units joined by a mechanical linkage and working with a longitudinally sawing or cutting unit which may be composed of saws or knife type cutterheads.
The purpose of this arrangement is to cause the workpiece (i.e. the two-faced cant) to follow, during its forward travel, a trajectory which will cause a suitable curving cut to be performed by the sawing or cutting unit. This result is accomplished by displacement, transversal to the feed axis, of preferably two moveable guiding units consisting of powered rollers clamping unto the horizontally parallel planar faces of the cant, in areas before and beyond the sawing or cutting unit, and continuously moving the cant laterally in opposite directions within those areas during its forward travel through the cut. These combined lateral motions are controlled in speed and direction by a computer interpretation of scanner data and they are further positionally interlocked by means of a control linkage connecting the two guiding units. The linkage effect, beyond insuring total motion synchronism, also establishes a fixed pivotal point for the workpiece, the location of which is determined by the geometry of the linkage itself. This latter feature of the system permits locating this pivotal point by design in the most favorable area to minimize lateral stresses to the cutting elements.
This preferred area lies between the cutting zone and the rotating axis of the saws or cutter heads so as to limit, on the one hand, lateral stresses on the cutting elements and, at the same time, keep any back rubbing of saw teeth or cutting knives at a practical minimum. It therefore becomes a matter of choice, within the scope of this invention, to locate the pivotal point for the workpiece somewhere between the cutting zone of the saw and its rotating axis, in order to reduce side thrust in said cutting zone and to strive at the same time towards equalizing side pressure on saw teeth in front and back. Both aims are worthwhile and can be reached with the present system, by adjustment of the workpiece pivotal point, through suitable proportioning of the linkage joining the two guiding units, as will be further explained in a detailed description of the diagram showing the general mechanical arrangements.
It is to be noted that, before entry to the processing unit (whether sawing or squaring), the cant must be prepositioned laterally and properly oriented in relation to the theoretical feed line. This is currently done in the industry by means of a number of infeed systems such as, for example, an apparatus commonly termed a xe2x80x9clinear optimizerxe2x80x9d and described in U.S. Pat. No. 5,429,161 to B. Allard of Jul. 4th, 1995.
Within the general concept of the present invention, curved sawing or cutting may be generated by three different methods of applications: by locating one guiding unit either before the cut or after the cut, or by using guiding units in both areas with a control linkage between the two. The main difference in results between these three options can be stated as follows: for a front location of the guiding unit, the latter part of the cut falls beyond its control and necessarily follows a straight line; for a rear location, the opposite occurs in that the first part of the cut is not affected by the guiding system and, therefore, is straight. In general terms, straight lengths of cut totalling approximately {fraction (21/2)} feet must be considered in either case, which means that the cut in an 8 foot log could only be curved for some 70% of its length, if only one guiding element were used. In the case of a system including a front and a rear mounted unit, suitably linked together, the total length of the cut may be curved if the computer solution demands it and the pivotal point of the cant can be maintained within the preferred area for most of the cut, whereby this pivotal point will remain ahead of the cutting zone in both cases where a single cant guiding unit is used.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that this detailed description, while indicating preferred embodiments of the invention, is given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.